The present invention relates to an extender for a thimble guide in a nuclear power plant, and more particularly to an extender which telescopically cooperates with the thimble guide to provide a shielded passage for the thimble right up to the bottom nozzle of a fuel assembly.
A typical pressurized water reactor 6 is schematically illustrated in FIG. 1 and includes a reactor vessel 7 which contains nuclear fuel, a coolant (water) which is heated by the nuclear fuel, and means for monitoring and controlling the nuclear reaction. The reactor vessel 7 is cylindrical, and is provided with a permanent hemispherical bottom and a removable hemispherical top. Hot water is conveyed from and returned to vessel 7 by a reactor coolant system which includes one or more reactor coolant loops 8 (usually two, three, or four loops, depending upon the power-generating capacity of the reactor, although only two loops are illustrated in FIG. 1). Each loop 8 includes a pipeline to convey hot water from the reactor vessel 7 to a steam generator 9, a pipeline to convey the water from the steam generator 9 back to the reactor vessel 7, and a pump 10. A steam generator 9 is essentially a heat exchanger which transfers heat from the reactor coolant system to water received at inlet 11 from a source that is isolated from the reactor coolant system; the resulting steam is conveyed via outlet 12 to a turbine (not illustrated) to generate electricity. During operation of the reactor 6, the water within the vessel and the coolant system is maintained at a controlled high pressure by pressurizer 13 to keep it from boiling as it is heated by the nuclear fuel.
Nuclear fuel is supplied to the reactor vessel 7 in the form of a number of fuel assemblies. Each fuel assembly includes a base element called a bottom nozzle and a bundle of fuel rods and tubular guides which are supported on the bottom nozzle. The fuel rods have cylindrical housings which are filled with pellets of fissionable material enriched with U-235. The tubular guides accommodate measuring instruments and movably mounted control rods of neutron-moderating material. A typical fuel assembly for a pressurized water reactor is about 4.1 meters long, about 19.7 centimeters wide, and has a mass of about 585 kg, and a typical four loop reactor might contain 196 such fuel assemblies supported parallel to one another on a core plate within the reactor vessel. After a service life during which the U-235 enrichment of the fuel assemblies is depleted, the reactor 6 is shut down, the pressure within the vessel 7 is relieved, the hemispherical top of the vessel is removed, and the spent fuel assemblies are replaced by new ones.
A number of measuring instruments are employed to promote safety and to permit proper control of the nuclear reaction. Among other measurements, a nuetron flux map is generated periodically, such as every 28 days, using data gathered by neutron flux detectors which are moved through a number of randomly selected fuel assemblies. To guide the flux detectors during their periodic journeys, closed stainless steel tubes known as flux thimbles extend through the bottom of the reactor vessel 7 and into the fuel assemblies which have been selected as measuring sites. This will be explained in more detailed with reference to FIG. 2.
In FIG. 2, a thick lower core plate 14 is horizontally mounted within reactor vessel 7, with reference number 15 identifying a portion of the hemispherical bottom end wall of the vessel 7. A number of fuel assemblies, including fuel assembly 16, are supported in an orderly array on plate 14. Fuel assembly 16 includes a bottom nozzle 17 having four legs 18 which are joined to a platform portion 20 with a centrally disposed aperture 22 in it. In addition to aperture 22, platform portion 22 has a number of flow openings 23 (only two of which are illustrated in FIG. 1B) to facilitate the circulation of water through fuel assembly 16. A number of fuel rods 24 are bundled together and supported on platform portion 20 at portions that are out-of-alignment with the flow openings 23. Within this bundle is an instrumentation tube 25 which is aligned with aperture 22 and which extends to the top nozzle (not illustrated) of fuel assembly 16.
A bore 26 having a threaded region 28 extends through core plate 14 in alignment with aperture 22. A conventional thimble guide 30, which may be configurated as illustrated in FIG. 2, is provided with a threaded portion and with a recessed wrench-engaging region 32 which permits technicians to screw guide 30 into threaded region 28 of plate 14 during fabrication of the reactor vessel 7. An annular groove 33 is positioned beneath region 32. After guide 30 is screwed into place, welds 34 are added for additional security. Typically guide 30 is 3.38 inches (8.58 cm) high, from the upper surface of plate 14 to the upper lip 35 of guide 30, and there is a gap of 1.37 inches (3.48 cm) between upper lip 35 and aperture 22.
A bore 36 extends through vessel wall 15 in alignment with bore 26. A vessel-penetration sleeve 38 having an outer diameter of about 1.5 inches (3.81 cm) extends through bore 36 and is welded at 40 to provide a seal which is resistant to high pressure. A bottom mounted instrumentation column 42 mounted on plate 14 extends between bore 26 and sleeve 38. Column 42 includes a fitting 44 which is attached to plate 14 by bolts 46, an upper pipe element 48 which is joined to fitting 44 by welds 50, and a lower pipe element 52 which is joined coaxially to element 48 at a tie plate (not illustrated). Lower pipe element 52 has an inner diameter of 2 inches (5.08 cm), so that there is a gap between sleeve 38 and element 52.
In a typical four-loop pressurized water reactor (having 196 fuel assemblies 16), 58 of the fuel assemblies 16 would be randomly selected for neutron flux monitoring. Accordingly, in such a reactor it will be apparent that there would be 58 guides 30, each communicating via a respective bore 26 and bottom mounted instrumentation column 42 with a respective vessel-penetration sleeve 38. During fabrication, sleeves 38 would be installed in the reactor vessel wall 15 and guides 30 and bottom mounted instrumentation columns 42 would be installed on core plate 14, the columns 42 being secured to one another by tie plates (not illustrated). Then the core plate 14 and attached structures would be lowered into the vessel, with the sleeves 38 fitting into elements 52. In the resulting structure, the upper ends (not illustrated) of sleeves 38 are spaced apart from the lower ends (not illustrated) of upper pipe elements 48, so that sleeves 38 are not in fluid-tight communication with bottom mounted instrumentation columns 42.
The bore 54 of upper pipe element 48 typically has a diameter of between 0.468 inches (1.189 cm) and 0.615 inches (1.56 cm) and terminates in a flared region 56. The bore 58 of fitting 44 is typically 0.68 inches (1.73 cm) in diameter and has flared regions at either end. The bore 26 typically has a diameter of 0.75 inches (1.91 cm). It will be noted that the channel provided by bores 54, 58, and 26 becomes progressively wider from upper pipe element 48, to fitting 44, to bore 26. This construction facilitates manufacture of the reactor and provides guidance for flux thimble 60 (to be discussed shortly) while avoiding the possibility that it might become struck in the channel.
Flux thimble 60 is a long stainless steel tube which begins at a plate (known as a seal table, not illustrated) outside the reactor vessel 7 and which has a closed end (not illustrated) that is normally disposed inside a fuel assembly 16. Thimble 60 slidably extends through tube 25, guide 30, bore 26, bottom mounted instrumentation column 42, and sleeve 38. A stainless steel guide tube (not illustrated) is welded to the outer end of sleeve 36, and thimble 60 extends within the guide tube to the seal table, which is typically located in a shielded position at an elevation near the top of vessel 7. Since the interior of the vessel 7 is in fluid communication with the interior of sleeve 38, it will be apparent that the guide tube provides a pressure boundary which extends around thimble 60 from wall 15 to the seal table, where a high pressure seal (not illustrated) is provided between the inner wall of the guide tube (not illustrated) and the outer wall of thimble 60. The net result is that thimble 60 provides a low-pressure access channel into the reactor vessel 7 from a shielded position outside of the reactor vessel 7.
A flux detector (not illustrated), about 2 inches (5 cm) long, is slidably accommodated within thimble 60 and is attached to a flexible push-pull cable (not illustrated) which extends through thimble 60 to flux-mapping equipment (not illustrated) located beyond the seal table (not illustrated). At periodic intervals, typically once every 28 days, the flux detectors are pushed to the tops of thimbles 60 and are then slowly withdrawn through the fuel assemblies 16 as flux measurements are taken at different heights to provide neutron flux map of the interior of the reactor vessel 7.
Normally thimbles 60 remain inserted in the instrumentation tubes 25 of the randomly selected fuel assemblies 16 between the periodic flux mapping operatons. Thimbles 60 must be withdrawn from fuel assemblies 16, however, at intervals of 12-18 months when the reactor 6 is shut down for refueling and fuel shuffling. During the refueling operation the nuclear reaction is terminated, the pressure within the reactor vessel 7 is relieved, and the guide tubes (not illustrated) are unsealed from the thimbles 60 at the seal table (not illustrated). The thimbles 60 (which are somewhat flexible) are then withdrawn by a distance of about 14 feet (4.27 meters) to free them frm the spent fuel assemblies 16, which are thereupon removed via remote control and replaced by fresh fuel assemblies 16. Thimbles 60 are then driven into the fresh fuel assemblies 16, the reactor vessel 7 and seal table are sealed, and power generation begins anew.
The conventional thimble guide 30 of FIG. 2 has several shortcomings. It has been found that considerable turbulence exists during operation of a reactor 6 in the region between the upper surface of core plate 14 and the lower surfaces of platform portions 20 of fuel assemblies 16. Guides 30 expose a significant portion of thimbles 60 to this turbulence, which may, depending on dimensional tolerances, vibrate thimbles 60 and increases wear to an undesirable extent. Simply increasing the length of guides 30 would be undersirable because fuel assembly designs may change, including the lengths of legs 18. Since guides 30 are permanently installed at the time the vessel 7 is built, any particular length for guides 30 that is selected at that time might make it impossible to take advantage of future design improvements in fuel assemblies. Even apart from this consideration, it would be undesirable to make solid guides 30 long enough to touch the bottom nozzles 17 of a particular fuel assembly design because very slight dimensional inaccuracies might upset the footing of legs 18 and leave the fuel assemblies tottering on top of one or more guides 30. Furthermore, it has been found that fluid flow in the gap around a thimble 60 due to the progressively widening channel from element 48 to fitting 44 to bore 26 may be sufficient to cause vibrations which increase wear.